Structure and sedimentation characterisation of sheared Mg(OH)2 suspensions flocculated with anionic polymers

In this study, magnesium hydroxide (Mg(OH)2) suspensions were flocculated using two polyacrylamide-poly(acrylic acid) copolymers with charge densities of 30% or 40%. Structural characteristics, including particle size distribution, shape and fractal dimension of the resultant flocs were investigated using complementary techniques; static light scattering, focused beam reflectance measurement, automated optical microscopy and cryogenic scanning electron microscopy. Sedimentation rates were analysed for 2.5 vol.% dispersions at various polymer concentrations and compared to predictions from a fractal modified Richardson-Zaki (FMRZ) settling model. FMRZ model predictions using the 90th percentile (d90) floc sizes produced the most accurate correlations to experimental settling data, as these larger flocs likely dominate settling dynamics by ‘netting’ smaller particles. Overall, the FMRZ settling model provided a first approximation of zonal settling rates, but when further examined by multivariate analysis, was found to be critically sensitive to small changes in fractal dimension.